Have you performed the 'self-test' routine found under the menu section?
If it shows your hours used is up around 2-3000 then the magnetron could be finished.
Bad [corroded] connections in the dome could also be a problem.
Do you have a users manual?
The link is above to download it.
That is the title of my manual, I don't know if I can upload it here.
I have never had your problem and no longer have that radar, just the manual.